1. Field of the Disclosure
The invention generally relates to quality control in the manufacturing and/or processing of a transparent sheet, preferably a glass sheet, and more particularly relates to the identification of imperfections or defects within the volume of the sheet.
2. Description of Related Art
In glass manufacturing, defect inspection is usually implemented by optical imaging methods. That is, defects in the material of a test object are imaged on a CCD chip by illuminating the test object, for example based on a bright field or dark field method, and using a lens system. The image so generated is evaluated in a computer unit, and based on the image information it is decided whether there is a defect in the glass, and optionally what kind of glass defect. Defect sensitivity of such systems depends on the pixel resolution, the resolution of the objective lens, and the signal-to-noise ratio of the recording unit.
The data rates of the image sensors which are typically implemented in form of line scan cameras are limited. A line scan camera having a horizontal frequency from 25 to 50 kHz, for example, can be employed with a feeding rate of the test object from 15 to 30 m/min. Usually one illumination channel is used. When using a plurality of channels, a plurality of camera benches would be needed accordingly.
An increase of the feeding rate of the production line would mean that with unaltered illumination less light would be available per image line, so that the signal-to-noise ratio would decrease. Alternatively, in order to avoid such deterioration, pixel resolution in the feeding direction could be reduced. This, however, would reduce the defect sensitivity.
In optical imaging systems, an increase in optical resolution causes a reduction in the depth of field, which should at least correspond to the glass thickness of the test object to be examined. Usually, even greater depths of field are required, due to feeding tolerances and because of a possible deflection of the glass.
Thus, for resolutions of less than 20 μm per pixel and feeding rates of 35 meters per minute, for example, imaging systems would become complex, difficult to control, and expensive.
Approaches for inspecting the skin or the volume of objects for defects are known in the prior art.
JP 4576962 B2 discloses a method for identifying defects in test objects, for example variations in thickness of a planar film due to a foreign body. For this purpose, light is irradiated onto the test object, and the backscattered light is analyzed. The analysis is based on an analysis of the polarized components of the backscattered light.
WO 2006/108137 A2 discloses an identification of defects in a glass sheet using a system which directs the light from a laser diode onto the glass sheet to be checked and evaluates the light backscattered from the front and rear surfaces of the glass sheet onto a line scanner as an interference pattern.
A problem of the prior art methods is that the line scanners used have a sensor area of approximately the dimension of the test object. Further, due to contaminations, the measured images frequently provide patterns without the presence of any defect in the test object, so that automatic evaluation of the images may be prone to errors. Furthermore, due to the sample geometry, patterns caused by minor defects may adversely be superimposed by a primary pattern, so that the defect cannot be detected reliably.